1. Field of the Invention
This invention relates to liquid polyorganosiloxane compositions that cure by a hydrosilation reaction to yield flexible, tough, and optically transparent elastomers.
2. Description of the Prior Art
It is known to prepare liquid or pumpable compositions by blending together a vinyl-containing polydiorganosiloxane, an organohydrogensiloxane curing agent, a platinum-containing catalyst and, optionally, a filler. These compositions can be cured under relatively mild conditions to yield elastomeric or resinous products, depending upon the types and relative concentrations of reactants present in the initial curable composition. The resultant cured products are useful for a variety of applications, including encapsulation of delicate electrical and electronic components, coating of various substrates, as pressure sensitive adhesives, and the formation of shaped articles by injection molding.
Reinforcing fillers such as fume silica have been used to improve the physical properties such as tensile strength, tear strength and modulus of cured polyorganosiloxane compositions.
The prior art teaches using resinous siloxane copolymers as a replacemnt for reinforcing silica fillers to improve the physical properties of cured polyorganosiloxanes while retaining the transparency of an unfilled material. Specifically, U.S. Pat. No. 3,284,406, which issued to Nelson on Nov. 8, 1966, U.S. Pat. No. 3,436,366, which issued to Modic on Apr. 1, 1969, and U.S. Pat. No. 4,472,470, which issued to Modic on Sept. 18, 1984 all disclose using resinous copolymers containing R.sub.3 SiO1/2, R.sub.2 ViSO1/2 and SiO.sub.4/2 units for this purpose. In the foregoing formulae R represents a monovalent hydrocarbon radical free of ethylenic unsaturation and Vi represents a vinyl radical.
The cured materials disclosed in the aforementioned Nelson and Modic patents typically exhibit hardness values of from 35 to 80 on the Shore A durometer scale, which is indicative of relatively hard, highly crosslinked materials. The aforementioned Modic patents teach inclusion of a finely divided nonreinforcing filler when transparency of the cured article is not a requirement. The curing agents taught by Modic are liquid polyorganohydrogensiloxanes containing at least two silicon-bonded hydrogen atoms per molecule, while the curing agents disclosed in the aforementioned Nelson patent contain at least three silicon-bonded hydrogen atoms per molecule. In addition, the compositions of Nelson contain 0.75 to 1.5 silicon-bonded hydrogen atoms per vinyl radical, which is equivalent to a maximum of 1.3 vinyl radicals per silicon-bonded hydrogen.
A means for increasing the tear strength of cured polyorganosiloxanes prepared from liquid compositions similar to those disclosed in the aforementioned Nelson and Modic patents while maintaining the viscosity of the curable composition within the range from 10 to 500 Pa.s is taught in U.S. Pat. No. 4,340,709 to Jeram and Smith, which issued on July 20, 1982. In accordance with the disclosure of this patent, 100 parts by weight of a liquid polydiorganosiloxane containing from 0.14 to 2.0 mole percent of diorganovinylsiloxy units is cured using the combination of a crosslinking agent and from 75 to 150 parts of a "coupler" containing from 3 to 9 mole percent of dimethylhydrogensiloxy units. The coupler, also referred to by those skilled in the art as a "chain extender", is a linear polydiorganosiloxane containing a silicon-bonded hydrogen atom at each of the two terminal positions of the molecule and no additional silicon bonded hydrogen. The crosslinking agents are defined as "hydride resins having only terminal hydrogen atoms or a linear hydride polysiloxane" containing hydrogen atoms only in the internal portion of each molecule on nonterminal silicon atoms.
In accordance with the claims of the aforementioned Jeram and Smith patent, the highest molar ratio of vinyl radicals to silicon bonded hydrogen atoms occurs when the repeating units of the polydiorganosiloxane are dimethylsiloxane units, this polymer contains 2.0 mole percent of dimethylvinylsiloxy units at the terminal postions, the composition contains 75 parts by weight of coupler per 100 parts of polydiorganosiloxane, and the coupler contains 3.0 mole percent of dimethylhydrogensiloxy units. Under these conditions the composition contains 0.012 vinyl radical per silicon-bonded hydrogen atom. This vinyl content becomes even lower when one includes the silicon-bonded hydrogen atoms present in the crosslinker.
It is therefore evident from the claims in the Jeram and Smith patent that the compositions contain more moles of silicon bonded hydrogen atoms than moles of vinyl radicals.
The cured compositions exemplified by Jeram and Smith exhibit hardness values of from 21 to 43 on the shore A durometer scale after curing for one hour at 100.degree. C. Hardness values in this range are desirable for protective coatings, encapsulating materials and certain types of molded articles.
The concept of using a chain extender containing two silicon-bonded hydrogen atoms per molecule in combination with a curing agent containing an average of in least three silicon-bonded hydrogen atoms to improve the tensile properties of cured polyorganosiloxane elastomers is taught in U.S. Pat. No. 3,697,473, which issued to Polmanteer et al. on Oct. 10, 1972. The chain extender is a polydiorganosiloxane wherein each of the two terminal units contains a silicon bonded hydrogen atom and no additional silicon bonded hydrogen is present in the molecule.
Both the chain extender and the curing agent contain a minimum of 10% of the available silicon-boned hydrogen atoms, and are the only sources of silicon-bonded hydrogen present in the composition.
The curable compositions described by Polmanteer et al. contain from 0.75 to 1.5 silicon bonded hydrogen atom for each vinyl radical present in the polydiroganosiloxane ingredient. In accordance with this teaching a curable composition cannot contain more than 1.3 vinyl radicals per silicon-bonded hydrogen atom. This is the same limit disclosed in the aforementioned Nelson patent.
The elastomers exemplified by Polmanteer et al. exhibit durometer values of from 11 to 38, measured on the Shore A scale.
The tensile properties and hardness values reported in the aforementioned Nelson, Modic, Jeram et al. and Polmanteer et al. patents indicate that the cured articles exemplified in this prior art do not exhibit the compressibility and resiliency required for some applications of polyorganosiloxane elastomers. One such application is as the light transmitting portion of flexible touch position sensitive optical screens of the type disclosed in U.S. Pat. No. 4,484,179, which issued to Kasday on Nov. 20, 1984.
Pressure sensitive optical screens and other types of compressible waveguides require a layer of cured, noncellular elastomer that is transparent, exhibits minimal light attenuation, and can be flexed or otherwise deformed by applying an electrostatic or mechanical force against at least one surface of the article. Polyorganosiloxane elastomers suitable for this application typically exhibit durometer values of 100 or less on the Shore 00 scale, and are considerably softer materials than those measured using the Shore A scale. In addition, the elastomer must be sufficiently resilient to resume its original shape almost immediately following release of the pressure which deformed it, and sufficiently tough to resist being punctured and/or torn when this pressure is repeatedly applied to the same area of the elastomer. It is also essential that the properties of the initially cured elastomer, particularly compressibility and resiliency, do not vary substantially with the passage of time.
If one of more surfaces of a deformable optical waveguide are overlayed with a film or membrane, the adhesion along this interface must be sufficient to prevent separation between the two layers and resultant void formation. The foregoing combination of properties required for optically transparent, deformable optical waveguides has not been disclosed for prior art polyorganosiloxane elastomer compositions.
Optically transparent, curable polyorganosiloxane compositions that include a vinyl terminated polydiorganosiloxane, a resinous organosiloxane copolymer containing dimethylvinylsiloxy, trimethylsiloxy and SiO4/2 units and two types of organohydrogensiloxanes are disclosed in U.S. Pat. No. 4,535,141, which issued to Kroupa on Aug. 13, 1985. One of the organohydrogensiloxanes corresponds to the chain extender of the aforementioned Jeram and Smith patent, and contains silicon bonded hydrogen atoms only at the terminal positions of a substantially linear polydiorganosiloxane molecule. The concentration of chain extender is equivalent to a concentration of silicon bonded hydrogen atoms that is at least 1.6 times the number of vinyl radicals present in the curable composition. The other organohydrogensiloxane functions as a curing agent and contains an average of at least 3 silicon bonded hydrogen atoms per molecule.
The present inventior found the cured elastomers prepared using the compositions described in the aforementioned Kroupa patent to be less than entirely satisfactory materials for compressible optical waveguides because the cured elastomer continues to harden with the passage of time and eventually becomes too hard to be compressed under typical operating conditions for the waveguide. This phenomenon is often accompanied by the appearance of hydrogen gas that interfere with the optical properties of the waveguide. The gradual hardening and generation of hydrogen are believed due to the reaction of the silicon bonded hydrogen atoms present in the elastomer with moisture.
Subsequent investigation has shown the waveguides containing the preferred elastomer compositions disclosed in the aforementioned Kroupa patent to have relatively poor compressive strength. These materials may therefore not be capable of withstanding the repeated compression experienced by a touch sensitive screen of the type described in the aforementioned Kasday patent.
An objective of this invention is to provide liquid or pumpable polyorganosiloxane compositions that can be cured by a hydrosilation reaction to yield optically transparent elastomers suitable for use as the light transmitting portion of a compressible optical waveguide.